Polycarbonate resins offer an excellent balance of properties with respect to transparency, toughness, dimensional stability and heat resistance. These properties make polycarbonate an ideal choice for the preparation of many types of molded, shaped or otherwise fabricated articles, especially including sheets or other structures and parts to be used in glazing and other outdoor applications. However, polycarbonates, like most organic polymers, degrade when they are exposed to ultraviolet (UV) light. As the polycarbonate absorbs significant amounts of high energy light and begins to degrade, it is known to become yellow and hazy and lose its toughness. Since polycarbonates derive much of their value and utility from their excellent optical properties, i.e. low color and high clarity, protection against UV becomes vital.
The use of various types of UV absorbers in the stabilization of polymers is well known. See for example U.S. Pat. No. 3,215,725 (bis cyano-diphenyl-acrylic acid esters); U.S. Pat. No. 4,812,498 (bis benzotriazoles); U.S. Pat. No. 5,288,778; GB 2,290,745 and EP 825,226 (triazine compounds); U.S. Pat. No. 5,821,380 (multifunctional 2-cyanoacrylic acid esters); EP 68,327 (cyclic imino esters also referred to benzoxazinones) and EP 110, 221 (benzophenones and benzotriazoles). These stabilizers function by absorbing incident UV radiation and dispersing the absorbed energy in a nondestructive manner. Their overall effectiveness in preventing UV degradation of the polymer depends on numerous factors, including absorptivity, compatibility, stability and distribution within the polymer. Their UV absorption effectiveness is a function of their concentration in the polymer, especially near the surface. Concentration of the UV absorber near the surface of the polymer is very desirable to prevent penetration of UV light and is considered to be more efficient and economical than dispersion of the UV absorber throughout the bulk of the polymer.
It is critical, therefore, for effective UV stabilization of polymers to have effective concentrations of UV absorbers present near the surface after processing and during long term. Both chemical and physical losses of the UV absorber will affect the concentration of UV absorbers in polymers. Chemical losses result from the thermal, photo-oxidative and oxidative reactions that inactivate or consume the compounds themselves. Physical loss of the UV absorber involves the removal of material from the surface by evaporation or dissolution that is not offset by its replacement in the surface layer by diffusion from the bulk polymer
When UV absorbers are physically lost from polymers, this may lead to undesired effects, such as fuming and plate-out in sheet extrusion or juicing and mold sweat during injection molding. All of these phenomena will result in reduced UV absorber concentrations in the resin and reduced production rates due to frequent, necessary cleaning operations of the equipment. Improved retention of an UV absorber conversely provides more effective stabilization in the desired end use as well as better processability in terms of reduced fuming, plate-out, mold sweat, juicing, etc.
Various methods have been used to improve the UV-stability of polycarbonate (PC). Common approaches are to use UV absorbers as additives in the polycarbonate and to apply layers or other surface treatments to prepare structures where the UV absorbers can be concentrated in the surface or outer layers to prevent UV radiation from deeper penetration into and degradation of the main thickness of the PC sheet. A number of methods and techniques have been developed to concentrate UV absorbers near or at the surface of polymeric materials. These include surface impregnation (see for example U.S. Pat. Nos. 3,309,220; 3,043,709; 4,861,664 and 4,937,026); coating a plastic article with solutions containing thermoplastic resins and UV absorbers (see for example U.S. Pat. Nos. 4,668,588 and 4,353,965); thermal bonding of film layers (see for example JP 07-9,560); and coextrusion (see for example EP 110,221; 247,480; 320,632; 338,355 and 825,226; GB 2,290,745 and U.S. Pat. Nos. 4,264,680 and 5,108,835). In these and other coextrusion references, there is an emphasis on the use of higher molecular weight and lower volatility compounds if used in higher concentrations in coextruded surface layers.
However, in the case of polycarbonate formulations and especially coextrudable compositions, which contain high levels of UV absorbers, it is always desirable to have improved combinations of physical, processing and appearance properties. It is especially desirable to have such improved resins, improved stabilized articles and improved processes where the stabilizers volatilize less and are better maintained in the compositions and articles during and after processing.